1. Field of the Invention
The present invention relates to a linear actuator, more particularly, to an improved linear actuator for translating a rotary motion resulting from a rotary motor into a linear motion.
2. Description of the Background Art
A conventional linear actuator for linearly operating a target object employs an electric motor or an oil hydraulic motor which has an rotating shaft. Various linear actuators are known, including a rack-and-pinion gear type linear actuator, a screw type linear actuator, a ball screw type linear actuator and a crank-arm type linear actuator.
With reference to the accompanying drawings, the constructions of the respective conventional linear actuators will now be described.
FIG. 1 is a schematic view showing the construction and operation method of a general pick and place apparatus adopting a linear actuator. As shown therein, the pick and place apparatus comprises a two-axis joint type mechanism including two rotational joints 11, 12.
In order to operate the apparatus, linear actuators 13, 14, such as a oil hydraulic cylinder are required which are extendable and retractable along their lengthwise directions.
FIG. 2 illustrate a conventional rack-and-pinion gear type linear actuator. As shown therein, the rack-and-pinion gear type linear actuator allows a rod 23 with a connection link 26 to be extended/retracted toward/from the linear actuator body, thereby satisfying the basic requirements thereof.
However, the rotation axis of a motor 25 for rotating the pinion 21 has to be perpendicular to that of the linear actuator, so that it is impossible to employ the same when there is a limitation with regard to the width of the linear actuator.
Besides, in addition to the space required for the rack 22 to reciprocate, an additional space is required so that the rod 23 at each end of the rack can make a reciprocating sliding movement therethrough, thereby disadvantageously increasing the size of the linear actuator body.
FIG. 3 is a schematic view of a conventional screw type linear actuator. As shown therein, such a screw type actuator satisfies the basic requirements of a linear actuator in principle, wherein a tube 31 having a female screw structure extends and retracts. However, friction between the female screw tube 31 and a male screw rotating unit 32 threadedly provided in the tube 31 lessens the efficiency of the actuator. In addition, when a load force F is imposed on the hinge at the connection link 36 and accordingly a bending moment M (M=F.sub.R S) is applied to an end portion of the tube 31, a normal force N (N=M/L) and a friction force F.sub.f (F.sub.f =.mu.N) which are applied to the screw threads of the male screw rotating unit 32 are increased, thereby incurring abrasion and stoppage due to locking, which may cause an interruption of the actuator.
FIG. 4 is a schematic view of a conventional ball screw type linear actuator. As shown therein, a sliding friction of the screw thread contact surface is converted into a rolling friction of balls 41 which circulate in and along the male screw body, so that the friction may be significantly reduced. However, in the ball screw type actuator, since a female screw movement unit 42 having a connection link 44 reciprocates between both ends of a fixed length of a male screw thread 43, this can cause a failure to satisfy a basic requirement that a linear actuator should be extended/retracted along its lengthwise direction.
Further, a coupling 47 for connecting a male screw rod and a radial bearing 46 which supports the male screw rod with a rotary motor 45 are additionally required, which thereby disadvantageously increases the production cost.
FIG. 5 illustrate a conventional crank-arm type linear actuator. As shown therein, the crank-arm type linear actuator is simple in construction. However, since the operating axis of the rotary motor 55 is perpendicular to the axis of a linear movement of the linear actuator, similar to the above-described rack-pinion gear type, and a space for the crank-arm 53 to rotate is additionally required, the width and height of the actuator are undesirably enlarged. In addition, the driving force of the rod 51 disadvantageously changes depending upon a rotational angle of the crank-arm.
In recent years, as shown in FIGS. 6A through 6C, a low friction linear actuator using a radial bearing instead of a ball screw was introduced. However, such a low friction linear actuator has disadvantages because it does not satisfy a basic requirement that a linear actuator should be extendable/retractable along its lengthwise direction in the same way as the ball screw type in FIG. 4.
In the construction of the actuator shown in FIGS. 6A through 6C, front and rear three radial bearings 63, 63', 63", 64, 64', 64" inscribed an inclination angle .alpha. on the rotational shaft 65, whereby the rotation of the rotational shaft leads to a linear movement of the movement units 61, 62 to which the radial bearings 63, 64 are fixed.
Here, the radial bearings 63, 64 are in contact with the rotational shaft with a predetermined force, so that the movement units are separated into an upper portion 61 and a lower portion 62 which are then compressed by springs 66, 66'. The compressive force of the springs is adjustable using screws 67, 67'.
The advantages of such a construction are that, since the movement units 61, 62 are linearly operated due to a spiral movement resulting from a rolling contact between the radial bearings and the rotational shaft, the friction force of the operation is significantly small, and even when the normal force increases due to the bending moment applied to the movement units, the operating efficiency has no significant change.
However, in this type linear actuator, the linear actuator does not satisfy a structural requirement that an actuator itself should be extendable/retractable in the same way as the ball screw type, because the radial bearings are designed to be inscribed on the rotational shaft. Further, the linear actuator according to this type is has an additional disadvantage in that a coupling 70 for connecting the radial bearings 69, 69' with a rotary motor 71 is additionally required.